1. Field of the Invention
The present invention relates to an optical recording medium to record and read information by means of a laser beam. More particularly, the present invention relates to an optical recording medium which is made up of a plastic substrate formed by injection molding, injection-compression molding, compression molding, or photopolymer process (2P process) and a reflecting layer of metal thin film formed on the substrate. The optical recording medium has a definite threshold value and is capable of stable recording over a broad range of recording power. In addition, the optical recording medium has superior recording stability and keeps the recording sensitivity unchanged over a long period of time.
2. Discussion of the Background
Optical recording media to record and read information by means of a laser beam are rapidly becoming practical on account of the recent development of fundamental technology on semiconductor lasers, recording materials, and film making processes and also by virtue of their ability to record a large amount of information. For an optical recording medium to be able to record information, a phase change leading to an optical change should take place at the part struck with a laser beam. Bubble making, pitting, and amorphous state-crystalline state transfer are some of the recording methods proposed so far.
An optical recording medium made up of a plastic substrate having a surface of minute roughened structure and a metal thin film formed on the substrate is known. Also known is an optical recording medium made up of a plastic substrate having a smooth surface and a metal thin film formed on the substrate. The advantage of the former is that recording is possible with a low power because the laser beam for recording is efficiently absorbed. This optical recording medium is disclosed in U.S. Pat. No. 4,616,237.
Conventional optical recording media have the following disadvantages. When recordings are made with a low recording power, the recording threshold value is not definite and the CNR (carrier-to-noise ratio) is gradually changed by the reading light. On the other hand, recordings with a high recording power form such large pits that cross-talk occurs between adjacent pits or cracking occurs in pits. All this leads to a decrease of the CNR. To avoid this, it is necessary to perform recording with a recording power in a narrow range. Meeting this requirement needs a more sophisticated drive unit.
Moreover, with the conventional optical recording medium, the recording characteristics change with time and the record stability is not satisfactory. In other words, when the recording medium is stored in an adverse environment, the recording threshold value and CNR change.